Encapsulated electrical coil having means to aid impregnation



H. REBER July 26, 1966 ENCAPSULATED ELECTRICAL COIL HAVING MEANS TO AID IMPREGNATION Filed July 16. 1963 Z .f m uuk@ M r -hu all llllllll Il www www@

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www@ effz W Tf. M me 0 vb Z IR im? im H w United States Patent 3,263,196 ENCAPSULATED ELECTRICAL CGIL HAVING MEANS T AID IMPREGNATION Hubert Reben', Zanesville, Ohio, assigner to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware Filed July 16, 1963, Ser. No. 295,372 4 Claims. (Cl. 336-96) This invention relates to electrical coils and more particularly to encapsulated electrical coils for stationary induction apparatus and to their method of construction.

Frequently electrical coilsrnust be protected so that they will be substantially unaffected by varying atmospheric conditions, mechanical shock, and moisture. It is common practice to accomplish such potection by encapsulating the coil in a thermosetting resin jacket. Frequently the encapsulating material does not penetrate completely into the interstices of the coil with the result that air remains entrapped in interstices within the coil. These air inclusions, or voids, may cause corona which undesirably increases the radio noise level of the coil and also reduces the impulse strength and otherwise impairs the electrical properties of the coil. This problem has been partially resolved by impregnating the coil with a liquid insulating dielectric or a thermosetting resin of low viscosity prior to the encapsulation. However, air inclusions may result even when the coil is both impregnated and encapsulated because of the difficulty with which the impregnating and encapsulating materials penetrate into the interstices within the coil.

It is an object of the invention to provide a new and improved electrical coil construction for stationary induction apparatus which, in comparison to prior art structures, offers less resistance to the penetration of impregnating and encapsulating materials into the interstices within the coil.

It is a further object of the invention to provide an improved method of constructing an encapsulated electrical coil which permits impregnating and encapsulating materials to penetrate into theinterstices of the core to a greater degree than prior art processes.

It is a further object of the invention to provide an improved method of constructing encapsulated electrical coils which results in fewer air inclusions and voids than prior art processes.

The word cylindrical is used in ythe description and the appended claims to connote an electrical coil having the form of a body generated by a straight line moving parallel to a fixed straight line and thus cover coils of circular, rectangular, oval, and also D-shaped cross section.

In accordance with the method of the invention, interleaved solid insulation sheets of high dielectric puncture strength and woven fabric insulation sheets are interposed between `concentric layers of conductor of a cylindrical electrical coil so that a fabricsheet is adjacent to each conductor layer and preferably to each solid insulation layer. The woven fabric sheets are capable of transmitting air and impregnating dielectric and thermosetting resin encapsulating material in liquid form therethrough. The coil may be first impregnated with a wax, an insulating oil, or a low viscosity unfilled resin prior to encapsulation, or the impregnation and encapsulation with resin can be accomplished in a single step. Thev resistance offered by the interleaved solid and woven fabric sheets to the penetration of the impregnating and encapsulating materials into the interior of the coil is considerably lower than with conventional insulation, and air bubbles entrapped between solid insulation sheets and underneath the conductor turns escape easily through and along the ice structed in accordance with the invention;

FIG. 2 is a side view. of the transformer of FIG. 1;

FIG. 3 is a radial section view through one of the coils of the transformer of FIG. l; and

FIG. 4 is a radial section view through an alternative embodiment of the invention.

Referring to FIGS. 1 and 2, an encapsulated electrical distribution transformer 1 is supported on a generally U-shaped, metallic hanger bracket 2 having horizontally disposed upper and lower legs 3 and 4 and a vertical crossbar S adapted to besupported on a pole.v An eyebolt 6 affixed to the upper leg 3 of the transformer hanger bracket 2 permits lifting of the transformer 1. A conductive ground pad 7 is secured to a depending member 8 affixed to the end of the upper leg 3 of the hanger bracket 2. Studs 9 on upper and lower core clamps 10 g upper and lower yokes 13 and surrounded by cylindrical coils 14 of rectangular cross section. Upper and lower core clamps 1t) are aixed to the magnetic core 11 by metallic banding straps 15 which encircle the core 11 and the core clamps 10, and the assembly of core and core clamps is then bolted to hanger bracket 2. The electrical coils 14 surrounding the winding legs 12 of magnetic core 11 are encapsulated within a thermosetting resin jacket 16. Magnetic core 11 may be closed and the coils 14 wound on the legs 12 thereof to form a core and coil assembly which is encapsulated as a unit. However, as illustrated in thedrawing magnetic core 11 preferably comprises two complementary U-shaped halves 17, the coils 14 are preformed and disposed side-by-side in `a mold while the encapsulating resin jacket 16 is cast around them, and the laminations 22 of the core halves 17 are inserted through the windows in the -coils 14 and abut in core joints (not shown) within the Windows in the coils 14. Crossover electrical leads (not shown) extending between the two coils 14 may be brazed together and encapsulated within jacket 16. A plurality of horizontally extending secondary bushings 18 may be molded integral with a portion 19 of the resin jacket 16 extending upwardly adjacent one face of the upper core yoke 13. Primary stub cables 2li having a suitable insulation covering such as polyvinyl chloride may have the conductor thereof swaged to the wire from the transformer primary winding and cast within upwardly extending projections 21 integral with resin jacket 16 on the side of core 11 opposite portion 19.

As illustrated in FIG. 3, the electrical coil 14 may be wound on a coil tube, or sleeve, 23 of suitable insulating material having high dielectric strength. If magnetic core 11 is closed and coils of circular cross section are wound thereon, the coil sleeve 23 may be built up of layers of shellac coated kraft paper wound to sufficient thickness and strength to pull wire thereon when the sleeve 23 is rotated. Alternatively, insulating sleeve 23 may be a tube of pressboard having layers bonded together by polyvinyl alcohol, or it may be constructed by scarng the ends of a sheet of solid pressboard and then joining the scrafed ends by a suitable adhesive such as cellulose acetate or polyvinyl alcohol. The sleeve 23 may be wound on the flanges of a pair of collapsible, multi-section gears (not shown) revolubly supported in surrounding relation to core Winding leg 12 in a manner well known in the art and disclosed in U. S. Patent 2,334,331 to Schultz et al. having the same assignee as the present invention so as to permit winding of a cylindrical layer of conductor thereon when the split gears and insulating sleeve 23 are rotated.

If the magnetic core 11 has a joint therein and coils 14 are pre-formed as illustrated in the drawing, insulating sleeve 23 may be formed on a rotatable mandrel and may comprise a few inner layers of a film of insulating material (not shown) which facilitates release of the coil from the mold after encapsulation, layers of glass fiber (not shown) in mat form to provide a moisture seal after encapsulation, and layers of kraft paper wound thereon to provide the necessary dielectric strength. Such layers of the coil tube 23 are molded Within the encapsulating jacket 16 and possess high mechanical strength after encapsulation of the coil 14.

The insulating sleeve 23 may be revolubly supported on a mandrel, a conductor of rectangular cross section secured thereto, and the insulating shell 23 rotated to pull an inner cylindrical layer 25 of rectangular conductor thereon constituting a portion of the transformer low voltage winding. A woven fabric sheet 26 of a suitable insulating material such as glass fiber cloth or cloth woven from insulating fiber material commercially available under the trademark Saran or the trademark Dacron is then wound in surrounding relation to the inner conductor layer 25. The fabric layer 26 will readily transmit air, the impregnating dielectric in liquid form, and the thermosetting encapsulating resin in liquid form therethrough. A cylindrical layer 27 of solid insulation sheet having high dielectric puncture strength is then wound over the fabric layer 26. The high dielectric strength solid insulation layer 27 may be of a porous or non-porous material but it preferably is of sheet fibrous material permeable to the impregnating dielectric in liquid form and to the polymerizable liquid resinous encapsulating material. For example, paper, glass paper, asbestos paper, or iber glass impregnated with resin is suitable for the invention, or the solid insulation sheet 27 may be composite insulation having mica flakes or glass flakes or polyester film bonded with a volatile adhesive medium to a sheet fibrous backing such as paper, cambric, or glass fiber and the like so that the composite insulation is permeable to both the impregnating dielectric and the encapsulation resin in liquid form. A fabric layer 28 similar to layer 26 is wound over the solid insulation layer 27, a solid insulation layer 29 similar to layer 27 is wound over the fabric layer 2S, and a second layer 30 of rectangular conductor continuous with the conductor of layer 25 is wound over the solid insulation layer 29 to complete the low voltage winding portion of coil 14. Five fabric layers 32, 34, 36, 38, and 40 interleaved with solid insulation layers 33, 35, 37, 39, and 41 are then wound successively over the conductor layer 30 to form the barrier between the high and low voltage windings of electrical coil 14.

A layer 43 of circular conductor forming part of the primary winding is then wound over the outermost solid insulation layer 41 by rotating the insulating shell 23. Three cylindrical layers of woven fabric 44, 46 and 48 interleaved with solid insulation layers 45, 47 and 49 are then wound over the circular conductor ylayer 43 so that the fabric layer 44 is adjacent the conductor layer 43. A second layer of circular conductor 50 continuous with the conductor of layer 43 is Wound over the outermost solid insulation layer 49. Three layers of fabric 52, 54 and 56 interleaved with three layers of solid insulation 53, 55 and 57 are then wound over circular conductor layer 50 so that fabric layer 52 is adjacent the conductor layer S0. A third layer of circular conductor 60 continuous with the conductor of layer 50 is then wound over the outermost solid insulation layer 57 to complete the high voltage winding portion of coil 14. Three layers of sheet fabric 62, 64 and 66 interleaved with layers 63, 65 and 67 of solid insulation sheet are then wound into rolls radially outward from the third circular conductor layer 60 so that fabric layer 62 is adjacent conductor layer 60.

After the winding mandrel, or the collapsible split gear means, is removed from within insulating shell 23, the coils 14 may be impregnated by completely immersing the coi-ls, either separately or as part of the transformer core and coil assembly, in an insulating impregnating uid such as wax, an impregnating oil, or a low viscosity unfilled thermosetting resin. Alternatively, the coils 14 may be impregnated and encapsulated simultaneously with a filled or an unfilled thermosetting resin. If the impregnating fluid is transformer oil or a low vis-cosity resin, the impregnating fluid may be permitted to drain from the coils for a desired period prior to encapsulating as desclosed in my copending application Serial No. 298,108, entitled, Method of Encapsulating Transformer, having the same assignee as the present invention.

Prior to impregnating moisture may be removed from the coils 14 by enclosing the coils 14 in a suitable sealed enclosure (not shown), drawing a vacuum on the enclosure and simultaneously heating the coil within the enclosure in a manner well known in the art to drive off the moisture from the coil insulation. The impregnati-ng material in liquid form may be introduced into the enclosure while the vacuum is still being drawn on the enclosure, and also the heating may be continued during the impregnating cycle.

It will be noted that layers of woven fabric 26, 32, 44, 52 and 62 are Vadjacent the conductor layers 25, 30, 43, 50 and 60 respectively. It will also be noted that both sides of each solid insulation layer is adjacent to a fabric layer, for example, solid insulation layer 27 is adjacent to fabric layers 26 `and 28 and solid insulation layer 47 is adjacent to fabric layers 46 and 48. The woven fabric layers readily transmit air and the impregnating and encapsulating Iliquids therethrough and, together with adjacent layers, form passages which permit air bubbles to escape axially of the coil 14 and offer substantially reduced resistance to the flow of the impregnating and encapsulating liquids axially of the coil during the impregnating and encapsulating cycles respectively. Inasmuch as a fabric layer is adjacent each conductor layer, air bubbles beneath the wire turns may easily pass through the fabric and escape axially of the coil. The impregnating and encapsulating materials in liquid form may enter the porous solid insulation layers from both sides through the adjacent fabric layers. Further, since fabric layers separate all solid insulation layers, air cannot become trapped between adjacent layers of solid insulation.

In the preferred mode of carrying out the invention illustrated in the drawing, the coils 14 are constructed Separate from the magnetic core 11, and the laminations 22 of core halves 17 are inserted through the Windows in the coils 14 after the coils are impregnated and encapsulated. A window between the coils 14 of transformer 1 facilitates cooling and increases the impulse strength of the transformer. The two coils 14 are assembled in the mold with crossover `connections made therebetween and stub cables :swaged thereto and are impregnated and encapsulated in the mold. If the impregnating material is oil or a low viscosity unfilled resin, it may be desirable to drain all impregnant from the coils after the impregnating stop except that retained by capillary attraction within the porous insulation of the coils. The impregnating material may be a wax or a low viscosity, unfilled, curable, or polymerizable, thermosetting resin such as an epoxy or ethoxyline resin. The invention also comprehends impregnation and encapsulation in one step with a filled or unfilled thermosetting resin.

Assuming that the impregnating and encapsulating cycles are separate, a liquid thermosetting resin such as an epoxy or ethoxyline resin containing an appropriate hardener and a suitable liller such as inorgani-c glass fibers, slate our, silica flour, or titanium dioxide is then introduced into the mold. The woven fabric layers permit the liquid encapsulating resin to penetrate radially therethrough into voids, crevices, and cracks within the coils 14, including the spaces between the wire turns and the interlayer insulation, and into the coarser pores of the solid insulation layers not filled by the impregnating dielectric after the draining step. The fabric layers in combination with the solid insulation sheet insulation olfer considerably less resistance, in comparison to known insulation, to the flow of the encapsulating and impregnating materials in an axial direction and permit the liquid resinous encapsulating material to penetrate into the innermost interstices within the coils 14 while permitting the air to easily escape through the fabric layers 4and thence parallel to the layers. The mold is allowed to cool to room temperature, thereby causing the encapsulating resin to solidify into enclosing jacket 16. The coils 14 may be baked in a suitable curing oven at an elevated temperature to convert the thermosetting encapsulating resin (and also the impregnant when impregnation is with a low vis-cosity unfilled resin) into an infusible solid. The coils 14 surrounded by resin jacket 16 are then removed from the mold and assembled with the laminations 22 of magnetic core 11. The core clamps 10 may then be secured to the core 11 with banding straps 15. The magnetic core and other exposed metallic elements may then be coated with a corrosion resistant protective cornpound, and the core and coil assembly including core clamps is bolted to hanger bracket 2.

Encapsulated coils constructed in accordance with the method of the invention are substantially free of air inclusions. It has been found that the impregnating and encapsulating materials penetrate to a much greater degree into the openings and interstices of the coil than prior art constructions. The greater ydegree of penetration of the impregnant and encapsulant reduces the radio noise level and substantially increases the impulse strength of transformer coils constructed in accordance with the invention in comparison to known methods of construction. The greater degree of penetration of the impregnant and encapsulantinto the' coil permits reduction in the thickness of the high-to-low voltage barrier and of the interlayer insulation `for a given voltage rating in comparison to known electrical coils and their method of construction. Further, the casting resin jacket 16 provides high mechanical strength preventing axial displacement of the conductor turns under short circuit conditions.

The embodiment of the invention illustrated in FIG. 4 is similar to ythat of FIG. 3 with the exception that layers of woven fabric are disposed on both sides of the conductor layers, and like elements in the two embodiments are given the same reference numerals with the addition of -the prime designa-tion for the elements of FIG. 4. For example, fabric laye-rs 31 and 32 are disposed above and below rectangular conductor layer 30', and fabric layers 4Z and 44 are disposed above and below the rst circular conductor layer 43. A greater degree of penetration of the impregnant and encapsulant is provided in the embodiment of FIG. 4 wherein the-fabric is disposed on both sides of the conductor l-ayers, thus permitting air bubbles beneath the conductor turns to escape and. the impregnant and encapsulant to enter in both directions through the adjacent insulation layers and from both ends of the coil.

While only a few embodiments of the invention and its method of construction have been illustrated and described, many moidcations and variations thereof will be apparent to those skilled in the art, and consequently it is intended in the appended claims to cover all such modiications and variations which fall within the true spirit land scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical coil including a plurality of concentric cylindrical layers of conductor, solid insulation sheets possessing high dielectric puncture strength and being permeable to an insulating dielectric in liquid for-m and to a thermosetting resin in liquid form disposed between said conductor layers, woven fabric insulating sheets capable of passing air and said dielectric and said resin in liquid form therethrough interposed between said conductor and lsolid insulation layers with a fabric sheet adjacent to each conductor layer and to given solid insulation layers, an insulating dielectric impregnating said solid insulation and said woven fabric sheets and the interstices of said coil, and a thermosettiug infusible resin enveloping said coil and lling the voids within said coil not impregnated by said insulating dielectric, whereby air voids are substantially eliminated from within said coil and the dielectric breakdown strength and radio influence noise level of said coil are improved.

2. An electrical coil including a plurality of concentric cylindrical layers of conductor, solid insulation sheets possessing high dielectric puncture strength and being permeable lto a polymerizable resin in liquid form disposed between said conductor layers, woven fabric insulation shee-ts capable of passing ai-r and said resin in liquid form .therethrough disposed between said. conductor and solid insu-lation layers with a fabric sheet adjacent to each conduct-or layer and to given solid insulation layers, and a polymerized infusible resin enveloping said coil and lling openings within said fabric and space between said conductor layers and said insulation sheets, whereby air voids are substantially eliminated wi-thin said coil and the radio influence noise generated by said coil is reduced.

3. An electrical coil in accordance with claim 2 wherein said solid insulation sheets and said woven fabric sheets are interleaved betweeneach adjacent pair of said conductor layers.

4. An electrical coil in accordance with claim 2 wherein said solid insulation sheets and said woven fabric sheets are permeable to an insulating dielectric in liquid form and including an insulating dielectric impregnating said woven fabric and said solid insulating sheets and the `interstices of said coil and wherein said resin fills the openings in said coil not filled by said dielectric.

References Cited by the Examiner UNITED STATES PATENTS 931,541 8/1909 Wohl et al. 336-206 1,795,052 -3/1931 Siragusa 336-96 1,799,011 3/ 1931` Fitzsimmons et al. 336-96 2,080,647 5/ 1937 Wiegand 264-272' 2,201,005 5/ 1940 Ford 336-185 2,363,329 11/11944 Horseld 264-272 y2,424,973 8/ 1947 Edmonds 336-197 X l2,490,106 '12/ 1949 Strickland 336-206 X 2,545,163 3/1951 Naster 336-100 X 2,935,668 5/1960 Robinson et al. 317-260 X l3,014,980 'l2/1961 Marshall et al. 174--121 LARAMIE E. ASKIN, Primary Examiner.

I. F. BURNS, R. K. SCHAEFER, Examiners. D. J. BADER, Assistant Examiner. 

2. AN ELECTRICAL COIL INCLUDING A PLURALITY OF CONCENTRIC CYLINDRICAL LAYERS OF CONDUCTOR, SOLID INSULATION SHEETS POSSESSING HIGH DIELECTRIC PUNCTURE STRENGTH AND BEING PERMEABLE TO A POLYMERIZABLE RESIN IN LIQUID FORM DISPOSED BETWEEN SAID CONDUCTOR LAYERS, WOVEN FABRIC INSULATION SHEETS CAPABLE OF PASSING AIR AND SAID RESIN IN LIQUID FORM THERETHROUGH DISPOSED BETWEEN SAID CONDUCTOR AND SOLID INSULATION LAYERS WITH A FABRIC SHEET ADJACENT TO EACH CONDUCTOR LAYER AND TO GIVEN SOLID INSULATION LAYERS, AND A POLYMERIZED INFUSIBLE RESIN ENVELOPING SAID COIL AND FILLING OPENINGS WITHIN SAID FABRIC AND SPACE BETWEEN SAID CONDUCTOR LAYERS AND SAID INSULATION SHEETS, WHEREBY AIR VOIDS ARE SUBSTANTIALLY ELIMINATED WITHIN SAID COIL AND THE RADIO INFLUENCE NOISE GENERATED BY SAID COIL IS REDUCED. 